(Meth)acrylic acid or (meth)acrolein has ordinarily been produced in a continuous manner by performing a gas-phase catalytic oxidation reaction on propylene, propane, isobutylene or (meth)acrolein by using molecular oxygen or a gas containing molecular oxygen in the presence of a complex oxide catalyst by means of a multi-tube type reactor having a plurality of reaction tubes each filled with a catalyst.
Since a retention portion, in which reaction tubes are not present, in a lead-out portion for a reaction gas in an outlet side of the multi-tube type reactor to be used in this method holds, although depending on specifications of the reactor, from 5 to 20% of an outer diameter of a reaction tube bundle, a volume of the retention portion has a size which can not be neglected compared with a flow volume of the reaction gas.
As a result, a problem is caused such that a reaction product in the reaction gas is decomposed in the retention portion to produce a by-product which, then, decreases a purity of (meth)acrylic acid or (meth)acrolein to be targeted or is changed into be a heavy by-product which, then, prevents the catalytic gas phase oxidation process from being continued or the like.
In Patent Document 1 (JP-A-5-125010), it is proposed that, by allowing a volume of the lead-out portion of the reaction gas in the outlet side of an oxidation reaction tube to be smaller than that of a lead-in portion for a raw material gas, a retention period of time in the lead-out portion for the reaction gas in the reaction tube outlet side, the retention period of time in the lead-out portion in the reaction tube outlet side is allowed to be short and, then, production of diketones as by-products is suppressed.
In Patent Document 2 (JP-B-62-17579), it is proposed that, in a downstream side adjacent to a catalyst layer in a reaction band region, a cooling portion comprising a filling layer of a solid inert material is provided. However, when the reaction gas is cooled, a heavy impurity having a high boiling point which is present in the gas is condensed to clog the reactor or the like and, then, an operation becomes sometimes incapable of being continuously performed.